1. Field of the Invention
The present invention relates to a four-terminal measuring device which measures a very small resistance or very small impedance of an object under test, and more specifically relates to a four-terminal measuring device which can measure extremely small regions of an object under test, or which can measure an object under test of extremely small size, by using nanotube terminals.
2. Prior Art
Conventionally, in order to measure the resistance Rx of an object under test, a method has been used in which a voltage is applied across both ends of the object under test, the current I flowing through the object under test and the voltage V across both ends of the object under test are measured, and then the resistance Rx is calculated using the formula Rx=V/I. A problem that has arisen in such cases is a generation of measurement error caused by the resistance of connecting wires such as lead wires, etc. and the contact resistance between the connecting terminals and the object under test.
FIG. 6 shows a conventional two-terminal measuring device which is used to measure the DC resistance of an object under test. Connecting wires 32 and 34 such as lead wires used for measurement, etc. are connected to connecting terminals C1 and C2 on both ends of the object under test 30, and a voltmeter 36, ammeter 38 and DC power supply 40 are connected to these connecting wires 32 and 34. The measured resistance Rm of the object under test is determined from the measured voltage V and current I as Rm=V/I=Rc1+Rx+Rc2+Rc, where Rx is the resistance of the object under test 30, Rc1 and Rc2 are the resistances of the connecting wires 32 and 34, and Rc is the contact resistance.
However, it is seen that the connecting wire resistances Rc1 and Rc2 and contact resistance Rc are admixed in this measured resistance Rm as errors. From the ratio of Rm/Rx, the error rate is given by (Rc1+Rc2+Rc)/Rx; accordingly, it is seen that the error rate increase abruptly as the resistance Rx of the object under test becomes very small. Consequently, the two-terminal measurement method is not suitable for the measurement of low resistance.
As a result, a four-terminal measuring device has been developed as a device for measuring low resistances. FIG. 7 is a circuit diagram of a four-terminal measuring device. Here, elements that are the same as in FIG. 6 are labeled with the same reference numeral, and only those elements that differ will be described. The connecting terminals C1 and C2 are respectively split in two, so that current terminals Ci1 and Ci2 and voltage terminals Cv1 and Cv2 are provided. Current connecting wires 42 and 44 and voltage connecting wires 46 and 48 are connected to these terminals as lead wires used for measurement. The resistances of the current connecting wires 42 and 44 are designated as Ri1 and Ri2, and the resistances of the voltage connecting wires 46 and 48 are designated as Rv1 and Rv2.
The effect as described in the following is obtained as a result of the provision of the current terminals Ci1 and Ci2 and voltage terminals Cv1 and Cv2. The current I flowing in the direction indicated by the arrow flows through the ammeter 38, current connecting wire 42, object under test 30, current connecting wire 44 and DC power supply 40, and thus returns to the starting point. Since the internal resistance of the voltmeter 36 is extremely large, there is almost no branching of the entreat I into the voltmeter 36; instead, the current flows xe2x80x9cas isxe2x80x9d into the object under test 30. Accordingly, the current I flowing through the object under test 30 may be viewed as being measured by the ammeter 38.
Meanwhile, since the internal resistance of the voltmeter 36 is far greater than the resistances Rv1 and Rv2 of the voltage connecting wires 46 and 48, the voltage drops caused by the voltage connecting wires 46 and 48 can be more or less ignored. Thus, the voltage V that is measured by the voltmeter 36 may be viewed as the voltage across both ends of the object under test 30.
Accordingly, the resistance Rx of the object under test 30 can be derived with good precision according to V/I from the current I and voltage V measured by the four-terminal measuring device. By changing from two terminals to four terminals, it becomes possible to ignore the connecting wire resistance and contact resistance. Thus, a four-terminal measuring device is an effective means to measure low resistances.
Such a four-terminal measuring device is certainly an effective device for measuring low resistances in oases where the object under test 30 is of a certain size or greater. FIG. 8 shows a resistance measurement diagram for a plate-form object under test in which a four-terminal measuring device is used. The constant-current power supply 43 is constructed by means of a DC power supply 40 and a constant-current resistance 41. The tip ends of the current connecting wires 42 and 44 (which are lead wires) are caused to contact the current terminals Ci1 and Ci2 of the plate-form object under test 30, and the tip ends of the voltage connecting wires 46 and 48 are caused to contact voltage terminals Cv1 and Cv2, which are disposed between the abovementioned terminals Ci1 and Ci2. Then, the resistance Rx of the object under test between the voltage terminals Cv1 and Cv2 is derived according to V/I. In this example, the current terminals Ci1 and Ci2 and voltage terminals Cv1 and Cv2 refer to the contact points where the current connecting wires 42 and 44 and voltage connecting wires 46 and 48 contact the object under test 30.
The current connecting wires 42 and 44 and voltage connecting wires 46 and 48 are lead wires whose tip ends are caused to contact the object under test 30. Accordingly, as the size of the object under test 30 becomes smaller, it becomes necessary to reduce the size of the lead wires. Even though the points of current terminals Ci1 and Ci2 shift somewhat in the right or left direction of the object under test 30, a constant current I flows between the voltage terminals Cv1 and Cv2. Accordingly, there is no great problem even if the current terminals Ci1 and Ci2 are in a state of surface contact with the surface of the object under test 30. However, because of precision requirements, the voltage terminals Cv1 and Cv2 must be single points in order to give both ends of the resistance that is being measured.
In other words, it is necessary to reduce the size of the tip ends of the voltage connecting wires. However, in conventional measuring devices, there are naturally limits to this size reduction. For example, even if the tip end of a metal needle is subjected to a sharpening treatment by means of electrolytic polishing, etc. or to an etching treatment using a semiconductor technique, it is difficult to reduce the size of the tip end to a nano-scale size. Meanwhile, as ultra-high densities have been developed in semiconductor techniques, objects under test have become progressively smaller. Today, for example, attempts have been made to manufacture nano-size electronic circuits, and resistance measurements and impedance measurements in the nano-region have become an absolute necessity. With conventional four-terminal measuring devices, it is absolutely impossible to handle such extremely small resistance measurements.
Accordingly, the object of the present invention is to provide a four-terminal measuring device that measures, with a use of extremely small nanotube, extremely small resistances or extremely small impedance values with good precision even in cases where the object under test itself is extremely small or the region being measured is extremely small.
More specifically, the present invention relates to a four-terminal measuring device that uses nanotube terminals and is characterized in that one or more of the tip end portions of the contact terminals of the four-terminal measuring device is formed by a nanotube.
Furthermore, the present invention relates to a four-terminal measuring device that uses nanotube terminals and is characterized in that the four-terminal measuring device is comprised of two current terminals which cause a constant current to flow from a constant-current power supply to an object under test and two voltage terminals which measure the voltage across both ends of the object under test; and in such a four-terminal measuring device, a nanotube terminal is formed by fastening the base end portion of a nanotube to a holder so that the tip end portion of the nanotube protrudes from the holder, and such a nanotube terminal is connected to desired terminals among four terminals.
In the present invention which is for a four-terminal measuring device that uses nanotube terminals, the nanotube terminal is connected to at least the voltage terminals among four terminals.
Furthermore, in the present invention which is for a four-terminal measuring device that uses nanotube terminals, the holder is a pyramid portion of an AFM cantilever.
In addition, in the present invention which is for a four-terminal measuring device that uses nanotube terminals, the constant-current power supply is formed by a DC power supply, and a voltmeter is formed by a DC voltmeter, thus measuring a very small resistance of the object under test.
In the present invention, further, which is for a four-terminal measuring device that uses nanotube terminals, the constant-current power supply is formed by an AC power supply, and a voltmeter is formed by an AC voltmeter, thus measuring a very small impedance of the object under test.